1. Field of the Invention
The present invention relates to a magnetoresistive element and to a thin-film magnetic head, a head gimbal assembly and a magnetic disk drive each incorporating the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments in read heads using TMR elements have been sought to conform to further improvements in areal recording density.
Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to a plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. In contrast, developments have been made for GMR heads having a structure in which a sense current is fed in the direction perpendicular to a plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element. A read head incorporating the above-mentioned TMR element has the CPP structure, too.
The CPP-GMR elements have great potential since the CPP-GMR elements have such benefits that the resistance thereof is lower than that of the TMR elements and that a higher output is obtained when the track width is reduced, compared with the CIP-GMR elements.
However, it is impossible to obtain a sufficient magnetoresistive change even if the configuration of layers making up the CIP-GMR element is directly applied to the CPP-GMR element. The major two reasons are as follows. One of the reasons is that, in the CPP-GMR element, the portion that contributes to a magnetoresistive change, that is, the portion made up of the free layer, the pinned layer and the nonmagnetic conductive layer, has a resistance that occupies a small proportion of the resistance of the entire element. The other of the reasons is that, in the CPP-GMR element, a magnetoresistive change is smaller, compared with the CIP-GMR element, the magnetoresistive change resulting from the scattering of electrons depending on the spin at the interface between the magnetic layer and the nonmagnetic layer (that is hereinafter called the interface scattering). That is, in an ordinary GMR element, there are two interfaces each formed between the magnetic layer and the nonmagnetic layer, wherein one of the interfaces is formed between the nonmagnetic conductive layer and the free layer, and the other of the interfaces is formed between the nonmagnetic conductive layer and the pinned layer. Nevertheless, in the CIP-GMR element, a sense current is fed in the direction parallel to the plane of each layer making up the GMR element, so that a sufficient magnetoresistive change resulting from the interface scattering is obtained. In the CPP-GMR element, in contrast, a sense current is fed in the direction perpendicular to the plane of each layer making up the GMR element, so that the interface scattering makes a small contribution to the magnetoresistive change in the GMR element.
In the CPP-GMR element, the scattering of electrons depending on the spin in the magnetic layer (hereinafter called the bulk scattering) makes a great contribution to a magnetoresistive change in the GMR element. Therefore, to obtain a great magnetoresistive change in the CPP-GMR element, it is effective to increase the thickness of each of the free layer and the pinned layer as the magnetic layers. However, if the thickness of the free layer is increased, there arises a problem that the direction of magnetization in the free layer is hard to change. If the thickness of the pinned layer is increased, there arises a problem that it is difficult to fix the direction of magnetization in the pinned layer firmly enough by means of the antiferromagnetic layer. Therefore, there is a limit to increasing the magnetoresistive change in the CPP-GMR element by increasing the thickness of each of the free layer and the pinned layer.
The Published Unexamined Japanese Patent Application 2003-152239 discloses a technique in which the number of interfaces that create interface scattering is increased by inserting a nonmagnetic metal layer to the free layer or the pinned layer so as to obtain a great magnetoresistive change in the CPP-GMR element. The Published Unexamined Japanese Patent Application 2003-152239 discloses a CPP-GMR element incorporating a free layer having a structure in which ferromagnetic metal layers of CoFeB and nonmagnetic metal layers of Cu are alternately stacked.
The asymmetry between the electric conductivity of the upward spin in the magnetic layer and the electric conductivity of the downward spin in the magnetic layer is indicated by bulk scattering coefficient β. To be specific, the bulk scattering coefficient β is expressed by the following equation where the electric conductivity of the upward spin in the magnetic layer is σb ↑ and the electric conductivity of the downward spin in the magnetic layer is σb ↓.β=(σb ↑−σb ↓)/(σb ↑+σb ↓)
Similarly, the asymmetry between the electric conductivity of the upward spin at the interface between the magnetic layer and the nonmagnetic layer and the electric conductivity of the downward spin at the interface is indicated by interface scattering coefficient γ. To be specific, the interface scattering coefficient γ is expressed by the following equation where the electric conductivity of the upward spin at the interface is σi ↑ and the electric conductivity of the downward spin at the interface is σi ↓.γ=(σi ↑−σi ↓)/(σi ↑+σi ↓)
The magnetoresistive change increases as the absolute value of the bulk scattering coefficient β increases. Similarly, the magnetoresistive change increases as the absolute value of the interface scattering coefficient γ increases. However, if the positive or negative sign of the bulk scattering coefficient β of a specific magnetic layer is different from the positive or negative sign of the interface scattering coefficient γ at the interface between the magnetic layer and the nonmagnetic layer, the magnetoresistive change caused by the bulk scattering and the magnetoresistive change caused by the interface scattering cancel out each other.
The bulk scattering coefficient β depends on the material making the magnetic layer. The interface scattering coefficient γ depends on the combination of the material making the magnetic layer and the material making the nonmagnetic layer. The bulk scattering coefficients β and the interface scattering coefficients γ determined for various materials are disclosed in Physical Review B, the United States, the American Physical Society, Sep. 1, 1999, vol. 60, no. 9, pp. 6710-6722.
The Published Unexamined Japanese Patent Application 2003-152239 discloses that the free layer preferably has a structure in which two to three ferromagnetic metal layers each having a thickness of about 1 to 2 nm are stacked, a nonmagnetic metal layer being provided between the ferromagnetic metal layers. This publication discloses a free layer having a structure in which ferromagnetic metal layers each having a specific thickness and nonmagnetic metal layers each having a specific thickness are alternately stacked. However, in such a structure, if each of the ferromagnetic metal layers is made to have a thickness of about 1 to 2 nm so as to have desired magnetic properties, there arises a problem that the thickness of the entire free layer is increased and the direction of magnetization in the free layer is hard to change. If the pinned layer has a such a structure that the ferromagnetic metal layers each having a specific thickness and the nonmagnetic metal layers each having a specific thickness are alternately stacked, there arises a problem that the thickness of the entire pinned layer is increased and it is difficult to fix the direction of magnetization in the pinned layer firmly enough by means of the antiferromagnetic layer.